Making bored and cast-in-situ piles using a hollow continuous auger is a method which has been known and exploited for many decades.
In brief, it consists in penetrating the ground in which it is desired to construct a pile with a continuous auger, the shaft of which is a hollow continuous cylinder, and then, during the inverse (extracting) movement of the continuous auger, in discharging concrete into the bored hole through the conduit formed by the shaft of the auger so as to gradually fill the space which the withdrawal of the auger tends to create.
In order to do this, the continuous auger is generally driven in a combined rotational-translational movement via a rotating engine which is itself capable of being displaced parallel to the axis of the pile which is being formed along a slideway mounted on a stand forming the general framework of the pile machine.
In a first stage (boring stage), the descending rotational/translational movement of the boring engine is generally maintained at substantially less than the value corresponding to the direct screwing in of the auger, with the pitch of the latter being taken into consideration, thus causing the earth along the enveloping surface of the auger to be cut away and the material loading the turns to circulate and rise.
In the second stage of forming the pile, once the specified depth has been reached, the continuous auger is extracted without rotation by a lifting translational movement of the boring head, with pressurized concrete being discharged simultaneously by means of a pump so as to gradually concrete the bored hole as the auger is withdrawn. Perfect and continuous filling can thus be ensured below the moveable plug formed by the turns of the auger themselves, loaded with excavated material
The concrete is conveyed into the channel formed by the hollow shaft of the auger through an injection head mounted, via a rotating fitting, either directly on the hollow spindle traversing the boring engine or, in the case of a solid spindle, laterally below the nose of the engine, at the head of the boring line.
The fundamental condition for correctly forming a pile by the hollow-auger method is that the concreting, and consequently the main channel formed by the hollow shaft of the continuous auger, must be maintained continuously.
It is therefore impossible to envisage forming a pile, according to the method described, by successively adding and removing auger elements in order to obtain the overall length of the structure, as is normally the case for making a conventional borehole by adding or removing elements of drilling rod.
This fundamental constraint therefore requires the use of a continuous auger for a length which is at least equal to that of the pile to be formed, employed as a one-piece element and, consequently, requiring a slideway for guiding the boring engine with a corresponding usable length.
Since this slideway must stand up to torsional forces corresponding to the very considerable driving torque required to drive the continuous auger in rotation (up to 20 rpm with the current machines), it assumes, for deep piles, large dimensions and a heavy weight which, as part of the same development, influence the features of the support machine. Furthermore, on-site movements of a machine of this type equipped with a long slideway often pose difficult problems of stability and safety.